The proposed experiments have two goals. The first is to identify the stimulus to the group III and IV afferents whose activation by muscular contraction signals the central nervous system that blood supply and demand in a working muscle are not properly matched. These afferents will be identified by showing that they respond more to an ischemic static contraction than to a normal contraction, even though both types of contraction develop the same tension. Once identified, the effect on the discharge of these afferents of arterial injections of several ischemic metabolites, such as adenosine, lactate, phosphate and possibly ammonia will be examined. Likewise, the effect on the discharge of these afferents of hypoxemia, acidemia and hypercarbemia will be examined. These experiments will be performed in unanesthetized decerebrate cats. Afferent impulse activity arising from the triceps surae muscles will be recorded from the dorsal roots. The second goal is to demonstrate in decorticate unanesthetized cats that group III and IV hindlimb muscle afferents are stimulated by a "true dynamic exercise", induced by activation of subthalamic and mesencephalic locomotor centers. Stimulation of these centers is known to cause a discharge pattern in alpha motoneurons that is identical to that evoked by dynamic exercise. The effect of ischemia, induced by occlusion of the arterial blood supply to the working muscle, on the afferents' response to dynamic excercise will be examined. In addition, the effect of infusing ischemic metabolites on the afferents' discharge will be examined. In addition to showing that group III and IV muscle afferents are stimulated by exercise, these experiments may shed new light on the puzzling question as to the nature of the metabolic stimulus causing these afferents to signal the central nervous system that blood supply and demand in a contracting muscle are mismatched.